DENDRITIC TUFA. 201 



clritic variety. This maj^ be taken as conclusive evidence that the lake 

 surface did not fall below the horizon of the thinolite terrace subsequent to 

 the deposition of thinolite until after the dendritic tufa was formed. 



DENDRITIC TUFA. 



A change in the chemical nature of Lake Lahontan, which terminated 

 the crystallization of thinolite, is recorded, by a third calcareous deposit, 

 which, from its resemblance in structure to arborescent forms, we have 

 called dendritic tufa. The conditions that brought about this change in the 

 character of the calcium carbonate precipitated appears to have been a 

 dilution of the waters in which the thinolite was deposited. This is evident 

 from the fact that the dendritic tufa reaches a hio-her level than the laver of 

 thinolite. This third variety is by far the most abundant of all the chemi- 

 cal deposits of Lake Lahontan, and occurs from the bottom of the basin up 

 to an elevation of 320 feet above the level of Pyramid Lake in 1882. In 

 many places it is not less than 50 or 60 feet in thickness and may at times 

 exceed this amount. The principal precipitation took place at an elevation 

 of between 200 and 300 feet above Pyramid Lake. The abundance of the 

 accumulation at this horizon is so great that it gives a convex outline to the 

 cliff's on which it was deposited, as may be seen at the Marble Buttes and 

 on the islands in Pyramid Lake. It is frequently suspended from cliffs in 

 pendent, comb-like masses, which sometimes suggest the appearance of 

 Cyclopean tile-work, and present surfaces of extreme ruggedness. The 

 aspect of cliff's when loaded down with tufa is well shown on Plate XXIV, 

 Vol. I, of the reports of the United States Geological Exploration of the 

 Fortieth Parallel, and is also represented on Plate XXXVI of the present 

 volume. Like the lithoid and thinolitic tufas already described, the den- 

 dritic variety is yellowish gray in color and weathers into similar rough 

 and angular forms. It is distinguishable at a glance, however, from the 

 earlier varieties by its peculiar dendritic structure. In typical specimens, 

 the sprays of tufa branch and expand from central nuclei, in such a manner 

 as to appear not unlike twigs of cedar changed to stone. This arborescent 

 or dendritic structure is shown on Plate XXXV, which is a reproduction of 



